1. Field of the Invention
The present invention relates generally to non-destructive evaluation of materials by ultrasonic methods, and specifically to the quantitative evaluation of the internal condition of composites by the measurement of Integrated Polar Backscatter from a composite material insonified by an ultrasonic transducer at a non-normal angle of incidence.
2. Description of the Related Art
In a known method for quantitative evaluation of the internal condition of a composite material, the composite is insonified by a single ultrasonic transducer at a non-normal angle of incidence, and a quantity called the Integrated Polar Backscatter is used as a measure of the condition of the composite. The Integrated Polar Backscatter is defined as the total energy of the backscatter signal detected by the transducer over preset frequency ranges, divided by the sum of the frequencies in the preset frequency ranges. The backscatter signals are usually normalized by comparison with the backscatter signal obtained from a reference object, such as a polished stainless steel plate in the same test setup.
When the composite material being tested has a smooth surface, the non-normal angle of incidence of the ultrasonic signal causes the reflected portion of the incident signal to be directed away from the transducer, so it does not contribute to the detected signal. The rest of the incident signal is refracted into the composite, where matrix cracking, porosity, inclusions, or other defects will cause a backscatter signal to be returned to the transducer. Integrated Polar Backscatter accordingly provides an accurate measure of the condition of a composite that has a smooth surface.
In practice, the surface of a composite material is not totally smooth, but has a regular surface texture caused by impressions from a "release cloth", which is a fine mesh cloth impregnated with teflon, used to keep the composite from sticking to the surfaces of the curing press. Such a surface texture causes some of the reflected ultrasonic signal to be directed back to the ultrasonic transducer, so the Integrated Polar Backscatter will have a constant component, independent of the condition of the interior of the composite. This surface backscatter obscures variations in the Integrated Polar Backscatter caused by internal defects, and can make the Integrated Polar Backscatter useless as a quality measure, unless precautions are taken to alleviate the effect of the surface texture.
The obvious remedy for a surface texture is to remove it. Grinding is not a useful method, but a strippable coating of a material with ultrasonic properties matching those of the composite can be applied to the surface of the composite to smooth out the surface texture and effectively eliminate the detrimental effect of the cloth impressions. The application of a coating before the ultrasonic evaluation, and stripping it off afterwards, are time consuming and expensive processes, however. Sometimes quality approval of both the coating material and the application and stripping processes would also be required, which effectively could rule this method out.
An alternate method for reducing the backscattering effect of the cloth impressions involves careful azimuthal alignment of the transducer and the composite material until minimal surface backscatter is obtained. A minimum in the surface backscatter is obtained when the incident signal is parallel to the impressions from either the weft threads or the warp threads in the release cloth. Generally, the surface backscatter has an absolute minimum when the incident signal is in-between the weft threads and warp threads directions. This alignment method can reduce the effect of surface backscatter to tolerable levels in most cases, but the azimuthal alignment is a cumbersome process that is difficult to automate.